The present invention relates generally to semiconductor integrated circuits, and particularly to a ring oscillator with voltage and temperature compensation.
Most integrated circuits need a timing device, or timer, to initiate certain operations regularly. For example, an integrated circuit of a dynamic random access memory (DRAM) equipped with a self refresh needs a timer to initiate the refresh operation. This timer measures the time interval between refresh operations, or elapsed time, to regularly trigger the refresh operation. There are many different designs of timers, but generally a timer for a refresh operation of a DRAM is usually constructed using a ring oscillator. The ring oscillator is usually fabricated on the same integrated circuit within the DRAM.
A typical ring oscillator mainly comprises an odd number of inverter stages connecting serially in a ring fashion with the output of each inverter connected to the input of the succeeding inverter in the ring. The output of the last inverter is connected to the input of the first inverter to produce an oscillating signal or oscillation frequency.
Those familiar with designs fabricated on integrated circuits know that the speed at which these designs operate is influenced by changes in supply voltage and operating temperature. Specifically, these designs operate at a higher speed with increasing supply voltage and/or decreased temperature and at a lower speed at the opposite extremes. This is largely due the changes in transistor conductance over voltage and temperature.
The oscillation frequency of the ring oscillator is influenced by the changes in voltage and temperature. When the temperature increases and/or the supply voltage decreases, the effective carrier mobility in the channel of the transistors decreases, the transistors become less conductive, and the speed of the inverter stages is reduced, which consequently contributes to the decrease of the oscillation frequency. At the other extreme, when the temperature decreases and/or supply voltage increases, the transistors are more conductive and the speed of the inverter stages is faster; therefore the oscillation frequency is increased.
For the reasons stated above, there is a need for an improved ring oscillator with a frequency stabilizing circuit so that the frequency of the ring oscillator is stabilized when there is a change in voltage and temperature.
The present invention is a voltage and temperature compensated oscillator frequency stabilizer.
In particular, the present invention describes an integrated circuit comprising a ring oscillator having a capacitor coupled between a complementary-metal-oxide-semiconductor (CMOS) inverter and inverter stages connected serially in a ring for producing oscillating output having rising and falling transitions. The oscillation frequency of the ring oscillator is set by the combination of the R-C time constant of the capacitor and the speed of the inverter stages. The ring oscillator is coupled to a frequency stabilizing circuit through a current discharge path. The main characteristic of the frequency stabilizing circuit is using the varying transistor conductances to compensate the conductance of the current discharge path in order to stabilize the oscillation frequency.
In one preferred embodiment, the frequency stabilizing circuit comprises a control circuit having p-channel and n-channel control devices and a current mirror designed to control the discharge rate of the current from the ring oscillator capacitor. The control circuit receives compensated voltage control signals from the output of a compensating circuit to allow the control devices to control the discharge rate of the current by using a current mirror principle. The compensating circuit comprises a current regulator coupled in series with a resistive element and a current limiter. In one preferred embodiment, the current regulator comprises a field-effect transistor. In an alternative embodiment, the current regulator comprises two or more field-effect transistors coupled in series. In yet another alternative embodiment, the current regulator is coupled to at least one field-effect transistor. The resistive element comprises a resistor connected in series with the current limiter. In one preferred embodiment, the current limiter comprises a diode. In an alternative embodiment, the current limiter comprises at least two field-effect transistors coupled in series. And in yet another alternative embodiment, the current limiter comprises at least one field-effect transistors coupled in series with a resistor. And in yet another alternative embodiment, the current limiter comprises a resistor. The conductance ratio of the resistive element and the current regulator device determines the necessary compensated voltage signals provided to the control devices of the control circuit to control the amount of current flowing through the control circuit to compensate and stabilize the oscillation frequency. Furthermore, the output of the compensating circuit is only suitable for the p-channel control device, therefore an inverting circuit having a pullup device and a pulldown device is designed to provide a control signal for the n-channel control device. In one preferred embodiment, the pullup device comprises a field-effect transistor and the pulldown device comprises two diodes coupled in series. In an alternative embodiment, the pullup device comprises two or more field-effect transistors coupled in series.
Since the frequency stabilizing circuit attempts to slow the ring oscillator based on the same conductance which inherently causes the ring oscillator to speed up, therefore the result is similar to negative feedback and has a stabilizing effect on the oscillation frequency.